The invention relates to an apparatus and method for determining the indentation hardness of a material, and more particularly, to an apparatus and method for determining the dynamic indentation hardness of a material.
Indentation hardness measurements have been used for several decades by machinists and researchers in various fields. Indentation hardness is a measure of a material""s resistance to penetration or permanent deformation. Indentation hardness measurements have been used to successfully monitor the effectiveness of processes such as heat treating, casting, forming, and welding. For the machinist, indentation hardness may indicate a material""s resistance to cutting.
Indentation hardness measurements are typically obtained by subjecting a material to a static indentation force with an indentor for typically 10-15 seconds, measuring the area of the indentation and then calculating the static indentation hardness using a test standard such as, for example, ASTM Test Method E92-82 for Vickers indenters which is set forth below:
Vickers Hardness (HV)=2Psin(xcex1/2)/d2=1.8544P/d2 wherein
p=load (kgf)
d=mean indentation diagonal (mm)
xcex1=face angle of diamond=136 degrees.
Although traditional static indentation hardness measurements provide insight into a material""s behavior during a specific process, these measurements do not accurately and sufficiently represent a material""s response during a dynamic event such as metal cutting or material removal during high speed machining. This is because deformation mechanisms of most materials are known to be rate dependent. For example, increased yield and flow stresses at higher strain rates in the case of metals, and increased fracture stress in the case of ceramics are rate dependent effects. Therefore, static indentation measurements do not completely represent a material""s behavior under dynamic events.
Many engineering applications involve dynamic loading conditions, e.g., machining, crashworthiness of automotive structures, impacts of space debris on space structures, and penetration resistance of armor materials. Existing hardness testers either measure only static indentation hardness or use a rebound technique to measure dynamic indentation hardness. The rebound technique to measure dynamic indentation hardness assumes constant yield pressure, neglects thermal effects and neglects sensitivity to indenter velocity therefore making the measurement of dynamic indentation hardness indirect and inaccurate. Moreover, the dynamic indentation hardness determined by the rebound technique cannot be compared to existing static indentation hardness values.
Accordingly, there is a need for an apparatus and method that can measure indentation hardness at strain rates similar to those encountered in actual dynamic processes. With the increasing use of composites in every realm of technology, there is a demand for quick and accurate determination of effective properties of these new materials. Dynamic indentation hardness measurements plays a key role in these activities.
The invention provides an apparatus and method for determining the dynamic indentation hardness of a material. The invention requires no prior knowledge of material characteristics which simplifies the measurement of dynamic indentation hardness. The apparatus and method utilizes a propagating stress wave to make a single indentation in the material. By suitably positioning a load transducer, the dynamic indentation hardness of a material can be determined accurately. The apparatus and method is useful in evaluating dynamic material response under high rate loading processes such as material removal during machining, dynamic wear, impact loading, and dynamic fragmentation. The present invention is of significant value in assessing a material""s ability to resist a specified dynamic load. Further, the invention can be used as a tool to quickly screen potential new materials to assess their suitability for dynamic applications. The obtained dynamic hardness can be directly compared to its static counterpart.
The invention provides for an apparatus and method for determining dynamic indentation hardness of a material using a propagating stress wave in a long slender bar. By mounting an indenter at one end and introducing a stress wave of suitable amplitude and duration at the other end, the invention delivers a single compressive stress pulse of required characteristics into the specimen material.
It is an object of the present invention to provide an improved apparatus and method for determining the indentation hardness of a material.
It is another object of the present invention to provide an apparatus and method for determining the dynamic indentation hardness of a material.
It is another object of the present invention to provide an apparatus and method for determining the dynamic indentation hardness of a material using loading pulses of less than one second.
It is another object of the present invention to provide an apparatus and method for determining indentation hardness having the load cell on the incident bar.
Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following detailed description, claims, and drawings.